Question
Consider the 3-bit block cipher in Table 8.1. Suppose the plaintext is 100100100.
(a) Initially assume that CBC is not used. What is the resulting ciphertext?
(b) Suppose Trudy sniffs the ciphertext. Assuming she knows that a 3-bit block cipher without CBC is being employed (but doesn’t know the specific cipher), what can she surmise?
(c) Now suppose that CBC is used with IV = 111. What is the resulting ciphertext?
Answer
(a) Consider the 3-bit block cipher in Table 8.1 from the text book. Suppose the plaintext is 100100100.
Convert 3-bit (three-three bits) block cipher is 100 100 100.
- Initially assume that CBC is not used.
- Change each three-bit input with the output 3-bit cipher as specified in the table 8.1 from the text book. Then the output for input plaintext 100 is 011.
So, the resulting cipher text is 011 011 011.
b) Suppose she knows that a 3-bit block cipher without CBC is being employed (but doesn’t know the specific cipher), then guess that the same as the three plaintext block.
c)
